This program is designed to develop and test methods for gene therapy to correct diseases of the immune system. The test disease will be the most common form (the X-linked form) of severe combined immunodeficiency (SCID), or ' Bubble Boy' disease. Bone marrow transplantation for SCID, while rescuing many patients, sometimes does not provide complete correction of the immune system. We have now developed retroviral vectors to carry a correct copy of the gene IL2RG (interleukin-2 receptor gamma chain, or common gamma chain) into cells from patients with SCID due to defects in this gene. The genes are transferred bone marrow stem cells, which are self-renewing for the lifetime of a person and can differentiate as needed into all the different types of blood cells. We have proven that we can obtain these stem cells from peripheral blood by giving patients injections of the cytokine G-CSF. After testing several vectors we found one with the best gene expression and ability to enter human blood-forming stem cells. We have tested and will continue to test these vectors in mice and human/sheep chimeras in which human stem cells grow into human blood cells that can be recovered from sheep. These studies have shown that it is likely we could help humans with SCID by doing gene therapy. A clinical trial for gene therapy for humans with X-linked SCID is being conducted at NIH in collaboration with NIAID. We will enroll patients for whom bone marrow transplant, the standard treatment, has not worked well. SCID is only one of many diseases that could be approached by gene therapy to blood-forming stem cells. Our studies with gene transfer for treating SCID are a pilot application chosen because of the special biology of the IL2RG gene. This gene confers a selective advantage on corrected cells as compared to cells without a functioning gene. Therefore if gene therapy is successful in SCID, the methods will be used for development of more general human gene transfer protocols that may be beneficial for patients with HIV/AIDS.